I will be printing some 6x6 negatives, and I'd like to include their borders in the print. The lab I'm using doesen't have any 6x6 holder with extra room, but the guy in charge recomended I use the 4 x 5 holder and sandwich it in their with cardboard or something. Anyone have any advice or tips on how I might do this?

If you buy some art card or mounting board, depending on what will fit best into your neg carrier, you can cut it to size and sandwich the neg into your 5x4 holder. Alternatively, you could use a 6x6 holder and file out the opening so that the borders would be shown in the print.